1. Field of the Invention
The present invention relates to reproduction of low frequency tones, and more particularly to an arrangement for reproducing bass tones within the passenger area of vehicles, namely automobiles.
2. Description of the Prior Art
The transmission of low and very low tone audio frequencies is accomplished by electro-acoustic transducers, commonly called loudspeakers or drivers. These transducers are arranged in an essentially closed loudspeaker enclosure so as to provide a good acoustic pattern for reproducing high quality, low frequency sound.
One type of enclosure for the production of a low frequency tone is a bass reflex enclosure. In a bass reflex enclosure, a loudspeaker is installed in an outer surface of a loudspeaker housing in a manner so that a front side of a diaphragm of the loudspeaker radiates directly into the volume of a surrounding environment. An additional radiation orifice is provided in the housing which acts as a phase angle rotator at a specific frequency range. The additional radiation orifice directs a resonance pattern of air volume and acoustic mass from a back side of the diaphragm of the loudspeaker so as to effectively add the acoustic energy radiated from the front and back sides of the diaphragm.
In another low frequency tone reproduction arrangement, a twin chamber enclosure arrangement, the interior of a loudspeaker enclosure housing is partitioned into two volumes which may be of equal or different size. A loudspeaker is fitted in the partition. Sound projection is effected through an opening that connects one of the two volumes with the outside environment. This arrangement can reproduce high quality bass tones with a smaller enclosure than a bass reflex arrangement. An acoustic filter effect is achieved with the twin chamber arrangement because the arrangement acts as a Helmholtz resonator having a second order band-pass transmission characteristic. The net sound from the twin chamber arrangement comprises only sound radiated by a bass reflex channel. By appropriate absolute and relative sizing of the two volumes, a very low, and within certain limits, an arbitrarily broad-band tuning can be accomplished in combination with a suitable loudspeaker. See L. R. Fincham, A Bandpass Loudspeaker Enclosure, KEF Electronics Limited, Maidstone, England. The bass tone transmission can be limited to an exceptionally low frequency spectrum by merely tuning the two different resulting resonance frequencies of the enclosure.
Thus, an advantage of the twin chamber enclosure arrangement is that it can be made considerably smaller than a bass reflex enclosure yet still provide the same sound quality. In situations where a sub-woofer is used with the stereophonic reproduction of sound signals, it is possible to dispense with the provision of a separate bass enclosure for each channel if the loudspeaker fitted in the partition is designed as a double-coil loudspeaker. Reproduction of bass audio signals by a sub-woofer enclosure is possible because such enclosures are fitted with a low-pass filter (cross-over) with a very low limiting frequency so as to filter out all but the low frequency audio signals for reproduction. The consequence of the low frequency filtration is that in this low frequency range, the human ear is no longer able to localize the source transmitting the low-frequency sound. Because the twin-chamber arrangement acoustically has second order band-pass transmission characteristics, highly effective frequency filtration is possible even with simple filters.
Like bass reflex arrangements, sub-woofer enclosure arrangements require at least seven liters of volume for an acceptable sound reproduction. Because this minimum volume requirement typically does not allow placement of the sub-woofer in an interior passenger area of a car, some car manufacturers place the sub-woofer enclosure arrangement in a trunk and feed sound outlet orifices into the passenger area. However, such placement can be expensive and because the sub-woofer enclosure arrangement is present in the trunk, the useful volume of the trunk is reduced.
Some sub-woofer enclosure arrangements are placed under the front seat of a car. There are some disadvantages to this positioning in that servomotors for electrical seat adjustment are also placed under the front seat. Even if there is space remaining for a sub-woofer enclosure arrangement, different enclosure housings have to be designed for each car manufacturer and each model of car in order to make optimum use of the residual space.